Since Indochina is 25 days and the other is 24 days, there is is a difference of $749AUD which covers an additional breakfast, dinner, one hotel night and single flight from cambo to vietnam. Would it really cost that much for this?

Looking at the timetable, taking the second option, you'll get to arrive a day early to the Vietnam highlight tour after a Asian Adventure.

What benefits is from taking the Big Indochina adventure in comparison to Asian Adventure+Vietnam Highlights?

Do the asian adventure and then the vietnam highlihts,,,,, stay the days betwen the tours in Siem Reap and the casa ankor hotel that is the same of contiki. You have no ideia how much I regret not doind that, cause the mekong delta SUCKS and Ho Chi Minh is to intense compared to all the nice things in Cambodia. Don´t waist your time in the mekong delta….